Lamps and displays using LEDs for illumination are becoming increasingly popular in many different markets. LEDs provide a number of advantages over traditional light sources, such as fluorescent lamps, including low power consumption, long lifetime, and no hazardous material, and additional specific advantages for different applications. For instance, LEDs are rapidly replacing Cold Cathode Fluorescent Lamps (CCFL) as LCD backlights due to smaller form factor and wider color gamut. LEDs for general illumination provide the opportunity to adjust the color or white color temperature for different effects. LED billboards are replacing paper billboards to enable multiple advertisements to timeshare a single billboard. Further, projectors that use LEDs as the light source may become popular in mobile handsets, such as smartphones, in the near future. Likewise, Organic LEDs or OLEDs, which use multi-colored LEDs directly to produce light for each display pixel, and which use arrays of organic LEDs constructed on planar substrates, may also become popular for many types of display applications.
LED lamps for general illumination and LED backlights for displays predominately use white LEDs, which typically comprise of a blue LED coated with a phosphor that absorbs most of the blue photons from the blue LED and re-emits lower energy photons over a relatively broad range of wavelengths. The resulting blended spectral emissions from the blue photons that are not absorbed by the phosphor and the phosphor converted photons can produce white light with various different color temperatures. Color temperature is a term used to describe the color of light as related to light produced by a black body radiator with different temperatures. For instance, so called warm white light with a color temperature of 2500 degrees Kelvin or 2500K has less optical power in the blue range, while a cool white light with a color temperature of 6000K has more optical power in the blue range. As such, the color temperatures of white LEDs can be adjusted by the thickness of the phosphor layer that absorbs more or less of the blue photons produced by the blue LED. Further different types of phosphors that re-emit photons over different wavelength ranges can also adjust white LED color temperature.
The well known CIE 1931 XY color space diagram illustrates all colors that humans can see in a 2-dimensional color space plot, which is represented in FIG. 70. The well known black body radiator curve is a curved line that runs though the 2-dimensional color space. Colors that are not on the black body curve, such as cyan, magenta, and green, cannot be assigned a color temperature. Since there are infinitely more colors not on the black body curve than on the black body curve and since phosphor thickness and emission spectrums cannot be precisely controlled, it is very difficult to produce white LEDs with a specific and defined color temperature.
Since the phosphor coating in most white LEDs convert high energy blue photons into lower energy photons in the green, yellow, and red range, the phosphor absorbs energy which reduces the optical efficiency of the LED and produces heat, both of which are not desirable. As such white LEDs with high color temperature and more blue content perform significantly better than white LEDs with low color temperature and significant red content. Consequently, some lighting manufacturers produce low color temperature lamps for general illumination that comprise both phosphor coated blue LEDs and red LEDs to produce white light with color temperatures in the 2500K range. Likewise, color filters in LCD displays that use white LED backlights typically enhance the relative red spectral content by various means including attenuating the blue and green spectral content. Additionally, backlights comprising white and red LEDs have been mentioned in the literature.
Although combining white and red LEDs to produce warmer color temperatures or more red spectral content improves performance parameters including energy efficiency and heat dissipation, controlling the color point is more difficult since the brightness of the red LEDs change differently with temperature, time, and other operating conditions than the white LEDs. As an example, U.S. Published Patent Application 2008-0309255 describes an approach for LED downlights from Cree. This application describes a lighting device comprising white and red LEDs, and an additional light sensor that is sensitive only to a portion of the spectrum produced by the combination of white and red LEDs. Additionally, such patent application further describes differential amplifier circuits and temperature sensors to control the color of the produced light. Not only do the extra components add cost, but since the light sensor is only sensitive to the shorter (bluer) wavelengths produced by the white LEDs, any variation in light intensity produced by the red LEDs other than variations produced by temperature changes, are not compensated. The most significant uncompensated variations are produced by LED aging, which causes the brightness of the LEDs and consequently the color produced by the lighting device to change significantly over time.
The LED industry, which includes LED suppliers such as Cree, Philips, Osram, Nichia, etc, has responded to the difficulty in producing phosphor coated LEDs with precise color points by introducing the concept of binning. As such, LED customers that order LEDs with a particular color temperature and brightness will receive LEDs grouped within particular color and brightness bins and labeled as such. To reflect the magnitude of the issue, which is very similar across all LED suppliers, Philips Lumileds' Luxeon Rebel datasheet reflects 19 different color bins for their cool white LED product and each color bin varies in the CIE 1931 XY color space by about 0.02 or more within each color bin. The well known McAdam ellipse, which arguably identifies the limits of human color discrimination, is on the order of ten times smaller. Additionally, only 5 of the 19 color bins for cool white Rebel comprise the black body radiation curve. For LED lamp and backlight makers that want to produce products with consistent color, binning is a major issue.
Although applications such as digital billboards and handheld projectors typically use red, green, and blue LEDs and not phosphor coated white LEDs, some of such applications could benefit from combining a white LED with RGB LEDs to produce brighter pictures. In the case of a digital billboard, each pixel could comprise a red, a green, a blue, and a white LED. In the case of a projector, the light source could comprise one or more red, green, blue, and white LEDs. Well known image processing algorithms enable the white LED to produce pixel or image brightness or grayscale while the red, green, and blue LEDs provide the pixel or image color components. Considering the complexity described in U.S. Published Patent Application 2008-0309255 to maintain color balance for just red and white LEDs, extending the traditional approaches to maintain color across red, green, blue, and white LEDs, is prohibitively complex and costly.
Many Active Matrix OLED (AMOLED) displays currently uses pixels made from organic red, green, blue, and white pixels. Since such LEDs are grown together on a single substrate, the matching between LEDs is much better than with inorganic red, green, blue, and white LEDs that may be manufactured at completely different locations, at different times, and with different processes. However, although the LEDs may initially match well, organic LED brightness degrades relatively rapidly with use, which can result in artifacts forming on displays that predominately produce the same image such as a computer background or toolbar. Such artifacts are visible and undesirable.
Among other reasons also, to eliminate or reduce the need for white LED binning, to enable red LEDs to be easily combined with white LEDs to produce warm white light and efficient display backlights, to enable white LEDs to be easily combined with RGB LEDs for billboards and projectors, and to eliminate artifacts burning into AMOLED displays, a need exists to easily and cost effectively set and maintain a precise color point produced by combinations of white and colored LEDs.